San Francisco Board of Supervisors President Matt Gonzalez jumped into the mayor's race a half-hour before filing deadline Friday, saying that he is the one progressive candidate with the best chance of beating the moderate front-runner, Supervisor Gavin Newsom.

Gonzalez's announcement set off a frenzy among left-leaning activists in a notoriously liberal city where even Mayor Willie Brown is considered a moderate. The race will share the ballot with some politically defining measures, including police reform, panhandling restrictions and increasing the minimum wage.

Supervisor Tom Ammiano and former Supervisor Angela Alioto, and to a lesser extent City Treasurer Susan Leal, had been trying to lock up the liberal vote, all running to the left of Newsom.

Former Police Chief Tony Ribera is running to Newsom's right, and Supervisor Tony Hall, another conservative who contemplated running, decided not to on Friday.

Now the question is whether Gonzalez, one of the Green Party's most prominent members, will further fractionalize the base or -- after the dust settles -- unify it.

As late as Thursday night, leaders in the gay and lesbian community backing Ammiano were trying to talk Gonzalez out of running.

"The more people on the left that run, the smaller of the pieces of the pie for each of them," said artist Debra Walker, a longtime Ammiano supporter.

Mayor Brown said he doubted Gonzalez would be able to coalesce the left and would help one candidate: "You would almost assume that Gavin Newsom is going to pay Gonzalez's filing fee."

But Carlos Petroni, publisher of San Francisco Frontlines, a leftist publication, said Gonzalez's candidacy should help the progressive cause.

"All three candidates, Gonzalez, Ammiano and Alioto, will be out on the street every day," he said. "The fact that there are three candidates will attract more people to vote."

And if it's between Newsom and a candidate to his left, Petroni predicted a united progressive front.

Medea Benjamin, a Green Party activist who had endorsed Ammiano but is now considering endorsing him and Gonzalez, said she hopes that after the anger and disappointment on the left dissipate, "the three campaigns in the end can work together to support the progressive who's the best vote-getter."

Gonzalez called Ammiano and Alioto "close allies whom I respect" but said their campaigns do not have the momentum to overcome Newsom.

"A number of progressives and moderates approached me to run because they felt the field lacked someone they could support enthusiastically," Gonzalez said.

While all the candidates are running to win the mayor's job, their first hurdle is to get into a runoff.

If Newsom continues to hold the lead, the race now is to see who will face him in a one-on-one contest in December. That could change if a new ranked- choice, instant-runoff voting system is put in place, but local elections officials say they don't have time to get it up and running in time for the mayor's race -- although that decision will be challenged in court Monday.

Alioto's take on Gonzalez: "Actually, Matt getting into the race makes me a moderate."

Ammiano, a former school board member who was elected supervisor in 1995 and faced Brown in a runoff for mayor four years ago, said the progressive community is not and has not been monolithic. He said he believes he still has the best shot at getting into the runoff "when people see the underlying record of longevity and breadth of issues."

Gonzalez, a first-term supervisor, represents District 5, one of the most politically liberal in the city that includes the Haight-Ashbury, the Western Addition and the Inner Sunset. His colleagues on the board voted to make him board president in January.

A former public defender, Gonzalez lost his one other attempt at citywide office when he ran for district attorney four years ago.

Gonzalez, who would speak only briefly to reporters after he filed his papers, said he is still getting his campaign in order and would have more to say about his candidacy later. But he offered that San Francisco "is fundamentally a progressive town" and that a candidate with progressive values and who can articulate them has a good chance of winning.

"In a one-on-one campaign with Newsom, I think we can win it," said Gonzalez, who has put has trial lawyer experience to use as a formidable debater.

In fact, he and Newsom squared off last fall over Newsom's Care Not Cash welfare reform measure, which voters ended up approving with nearly 60 percent support. Now, while Care Not Cash is back at the Board of Supervisors for consideration after a judge said the issue shouldn't have been decided by voters, Gonzalez has taken on Newsom again.

Followed into the elevator by reporters after he left the basement elections department and headed back to his second-floor office, Gonzalez expanded some on what progressive values he'll push:

Gonzalez was behind a measure for the November ballot to raise the minimum wage in San Francisco to $8.50 an hour.

All the major candidates have attached themselves to ballot measures: Alioto's calls for revamping the city's homeless shelters; Ammiano's two would beef up police accountability and give the city more financial stability in tough times; Newsom's would restrict panhandling; and Leal's calls for helping working families pay for pre-school child care.

By entering the race late, Gonzalez lags his opponents in putting together a field campaign and raising money. He is expected to get a lot of help from the Green Party, which already has been playing him up nationally. But San Francisco is still a town dominated by Democrats.

The one factor in the race that none of the campaigns had counted on was the recall election of Gov. Gray Davis, which has eclipsed anything local in the media and voter attention.

"We just hope there won't be voter fatigue because of the recall," Ammiano said. "It looks like Arnold and Arianna are overshadowing our local races."